1. Field of the Invention
The present invention relates to an image processing method, image processing program, and an information processing system, and in particular to image processing executed when an ink-jet printing head forms and outputs an image.
2. Description of the Background Art
In recent years, conversion of information to digital electronic format has accelerated, and accordingly, a printer, a facsimile, and an image processing system, such as a scanner, etc., which digitalizes and outputs an image of a document, have become indispensable. Increasingly, these image processing systems are that are used as copiers, printers, facsimile machines, and scanners or the like, and equipped with imaging, image formation, and communications capabilities, etc. In recent years, conversion of information to digital electronic format has accelerated, and accordingly, a printer, a facsimile, and an image processing system, such as a scanner, etc., which digitalizes and outputs an image of a document, have become indispensable. These image processing systems frequently constitute a multiple-functional machine used as a copier, a printer, a facsimile, and a scanner or the like including imaging, image formation, and communication capabilities, etc.
As one example of such an image processing system used to output digitalized information in this way, there is a type of printer that employs an ink-jet method (hereinafter, simply referred to as an ink-jet printer) and includes one or more printing heads provided with ink ejection nozzles. The ink-jet printer applies pressure to an ink liquid chamber disposed in the printing head using a PZT (PbZrO3—PbTiO3) element or a heating element, etc., and ejects ink droplets from the nozzles in the head onto a printing medium, thereby performing image formation.
The ink-jet printer executes image formation by ejecting ink at each pixel constituting an image, based on image data converted to a given resolution. Depending on the specifications thereof, the ink-jet printer may be one that represents each pixel with only a single gradation and distinguishes only between color and colorless, or one that represents each pixel by three gradations of heavy, normal, and light in addition to colorless.
By contrast, each pixel of image data used to generate an output image generally has multiple gradations that exceed the ability of an ink-jet printer to express them. Therefore, when an image is formed and outputted by the ink-jet printer, the original image data that is expressed in multiple gradations is converted into gradations adapted to the capabilities of the ink-jet printer, and halftone processing is carried out so as not to degrade the quality of the image when viewed.
As an example of halftone processing, an error diffusion method and a dither method are known. In addition, a DBS (Direct Binary Search) technique that directly seeks out error and processes it has also come to be used in recent years, as typified by Japanese Patent Application Publication No. 2010-215399 (JP-2010-215399-A).
However, there is a problem with the ink-jet printer in that an image is not always formed as intended due to inconsistency in the performance of ink ejected and settled on a sheet (hereinafter, simply referred to as ink droplet landing performance). As used herein, the term “droplet landing performance” includes but is not limited to size, position, shape, etc., of the droplets of ink ejected from the nozzles in the printing head.
To solve problems caused by this kind of variation in droplet landing performance, a method of halftone processing has been proposed that substantially reduces a driving frequency when discharging either droplets having a size causing uneven ejection from the nozzles or nozzles exhibiting uneven ejection, as described in JP-2008-126453-A. An added complication is that, in recent years, a line-type ink-jet printer employing multiple heads arrayed in a lengthwise direction is sometimes used for improved productivity, because a wide image can be formed at a single scan.
However, since such a wide image formed all at once requires the use of many nozzles, the problem of the above-described variation in droplet landing performance is exacerbated. More specifically, because the image needs to be outputted in a single scan, the nozzles aligned on each scanning line are necessarily densely packed, and accordingly a variation in droplet landing performance between the nozzles more easily degrades the quality of the image.
Although halftone processing is performed in accordance with the droplet landing performance as described in the method of JP-2008-126453-A, a reduction of the driving frequency of a driving waveform tends to result in a blurred output image.